Cannot access Admin Backend

I’m working on a magazine website based on WP. The backend was up and running smoothly until 2 hours ago. I don’t know why but all of a sudden, I couldn’t reach it. Used both http://www.mywebsite.com/wp-admin/ and http://www.mywebsite.com/wp-login.php but no success. When I tried the first one “wp-admin”, all I see is a blank screen. With the second method, I can see the log in screen but it doesn’t log me in and just shows a blank screen as well.

I’ve browsed through support forums and have disabled all the plugins by replacing the content of active_plugins table with a:0:{}. After that backed up and then deleted all the plugins from wp-content/plugins folder. I even re-downloaded and uploaded WP files. Beside I cleaned cookies & cache several times, and tried with different browsers. Still no success and cannot reach backend at all.

However, I can still view the front end and browse through the website without any problem.

I’m suspecting couple plugins that were installed. They are WP Optimize, Secure WordPress, and WP Security Scan. I cannot say why exactly, but they are suspects in my eyes.

As a note, before I had this problem, I had HTTP Error 500 Internal server error several times while working on the content via backend. And even received blank screen while updating/editing content. Was that an indication of this problem?

I do urgently need help because we are planning to launch the website by tomorrow night. Please someone help!